Observations - Looking at the FRS transmission shop manual (see post 461 by Bob Bundy):

(Warning long post… apologies – too verbose)

FRS transmission is very similar in basic configuration to the Miata 6 speed (e.g. location of each gear within the transmission including the unique use of single synchro hub for 5th & Reverse with the 6th gear set in the extension housing). The Honda variant of this family of transmissions is very different and appears to have little in common with the Miata or FRS 6 speed.

The internal 5 –6 gear interlock/gear change linkage has moved from the main housing into the extension housing. On the Miata 6 speed, it is the removal of the pivot bolt (located in the middle housing top right) that forces a transmission disassembly to repair. It looks like interlock/gear change linkage in the FRS design eliminates this potential issue but requires more volume in the extension housing… Other drawings of the shift mechanism look similar to the Miata but, any small evolution in the design can prevent easily moving parts from one transmission version into the other.

Disassembly of both transmissions would be needed to confirm. External shifter linkage is totally changed so this may impact the ability to use the Miata extension housing.

Other FRS 6 speed differences: bolt pattern on bell housing, location of clutch fork pivot and arm hole in bell housing, rear mounted / reverse rotation starter, longer clutch shaft (first motion), no power frame mount, use of a remote shifter, … also noted by others, increased input and output shaft diameters.

One option is to install the FRS transmission as a complete assembly using an engine to transmission adapter plate. The longer clutch shaft and deeper bell housing may allow room for a relocated or custom clutch release fork and throw out bearing. A clutch release cylinder mount would also have to be fabricated (mounted to the adapter plate?). The adapter plate would also provide something the Miata starter to bolt to.


Now for some arm chair engineering… Comparing the FRS to Miata 6 speeds:

If the high torque failures in the Miata 6 speed are related to transmission shaft bending (see “another 6 speed bites the dust” post # 305 page 16 – gear pressure angle), then an increase in shaft diameter will reduce the amount of counter shaft and 3rd motion shaft deflection and “may reduce” this deflection and the resulting gear tooth failure. Increasing the shaft diameter about 5% would reduce the deflection about 21% (increasing stiffness is on the order of R^4). Any increase in shaft diameter is a good thing…

A reduction in gear helix angle would reduce the drive gear to driven gear contact pressure and also reduce the axial thrust (end) loading on the shafts and transmission case bearings. Lower helix angle is a good thing…

Wider gears also help but may not be achievable if the center housing is the same or similar dimension. Any % increase in active gear width would increase the strength to the same % amount. Wider gears are a good thing…
As was already pointed out (Bob Bundy), the main and counter shaft distances are believed to be unchanged between the FRS and Miata transmissions so, there is no opportunity to improve that factor… we are SOL on this one...


Things to compare and measure between FRS and Miata 6 speeds:

Shaft diameter on the 3rd motion shaft (shaft diameter inside the bearings for – the end of the shaft that fits into the bearing in the 1st motion shaft, reverse, 4th, 3rd, 2nd and 1st)

Shaft diameter on the counter shaft (2nd motion shaft) for 3rd and 4th
Helix angle for each gear pair – Counter shaft drive pair, 4th, 3rd, 2nd and 1st gears.

Transmission housing bearings supporting the 1st, 2nd and 3rd motion shafts will likely be larger also… It’s good to know what the new sizes are, but likely not as critical as the others.

I assume 5th gear (direct 1:1) and 6th do not fail…



How to break a transmission (especially gear teeth), drive shaft, CV joint, Diff, axle? … Torsional shock loading is one failure mode. The main issue is moving from a low load to very high torque spike as the tire spins up (when air born) then rapidly slows down to road speed once it finds traction. Wider sticky tires make it worse as the increased grip increase the height of the torque peak.

Torque spikes may be caused by axle hop, spinning up an unloaded drive wheel when jumping from curbs at the track, dumping the clutch (e.g. a drag race start), … etc. Axle hop is likely worse (lots of high torque peaks in a very short time) but torque spikes for any reason can cause damage.

 

This Evasive Motorsports blog entry prior to the PPIHC says it is a Hewland. Link: ? frs

 

Once the trans is apart I will measure all of this. Surely someone else here has a 6spd that is opened up and can get some numbers. I'm not terribly inclined to pull apart my (known good) 6spd to make comparative measurements, but if that's the only way to crack this nut I'll consider doing it.

 

so how doe these ratios look with the 4.3? http://www.summitracing.com/parts/fm...6266/overview/ 6th gear with the 4.3 is about the same as the stock tranny would be with the 3.3.

 

With a 4.3, the drops are a little long, but it's workable.

 

Assuming shifts around 7100rpm..
2-3 shift drops to ~5200rpm
3-4 shift drops to ~5500rpm
4-5 shift drops to ~5700rpm

Low power or really peaky motor may need a tad shorter, and on the other end of the spectrum I'd probably want a longer 2-3 drop for something with gobs of torque (non-BP) but for most turbo/rotrex miata setups with 200+whp I'd think that would work pretty well.

-Ryan

 

The first gear combination is just a little taller than a 5-speed and a 4.10 first gear, which is probably great.

The trans is large and a bit heavy, but if it can fit in the tunnel and have decent shifter placement, it would be bulletproof.

 

That's a T56, basically the same tranny used in the typical LS V8 swaps. The tunnel has to be widened for those applications but they fit fine after that. That would be sick if someone made a kit to adapt one for BP engines.

 

I'm still running a TII box with no problems. Running a shorter rear end would make it better. I'm making around 350whp and drifting on 245/17's and I haven't had any problems in a year and a half

 

I believe its actually a T6060, which is more or less an upgraded version of the t56. I seem to have mixed up the parts I was looking at though, I had been looking at trannys with the same bellhousing pattern as TKO500 which should bolt to the lakewood housing and didnt realize I'd moved over to things using the t56 pattern.

 

BRZ trans came apart last week. I grabbed a few pics and some measurements. There are full res versions in the Dropbox link.

https://www.dropbox.com/sh/4zulo8pwe...rZfD5Uj9a?dl=0

Needle bearing inner race (shaft) measurements:

4th gear has no needle bearing, it rides on an odd grooved race that slips onto the countershaft (see pic): OD of the race is 46mm, ID of the gear is a fraction of a mm larger
3rd gear 38mm
2nd gear 43mm
1st gear has a collar/race that slides onto the shaft: collar/race OD 40mm, collar ID/shaft OD 32mm
Input shaft/output shaft interface (where the input rides on the output) 19mm
Both large shaft support bearings are 75mm OD, smaller ones are 52mm and 51mm OD. Didn't measure ID's.

 

Don't know if you figured it out already but in the comments of that video Evasive said they were running a "hewland dog box"

 

If anyone needs 5 spd Solstice/Sky Ar-5 transmission dimensions, I have one and can pull tape for approximate distances.

 

Interesting I’ve been pulling apart transmissions in the process of rebuilding a broken Quaife 5speed. Observations that the Miata 5 speed has puny gears compared to the Miata AZ6. Even the Quaife gears are significantly smaller diameter and don’t have near the width as in the AZ6. I’m surprised if the Quaife is really much stronger than a 6 speed unless the gear material is really way better. The only advantage I can see with the 5 speed in terms of strength is it has more shaft bearings and less span between bearings to support the shaft spacing. Has me thinking if somebody would just make a Quaife like gearset for the AZ6 Miata box it would be significantly stronger than the Quaife 5 speed.

Also somewhat interesting is taking apart 5 speeds from various years it is interesting seeing some fairly significant design changes. Reverse went from straight cut to helical cut. OEM center bearing on the secondary shaft is a roller bearing on NA 5 speeds. It’s a ball bearing on the Quaife that came from FM. Looks like they switched to a sealed ball bearing in the NB 5 sp? I had the rollers lock up on a couple of my broken 5 speeds. I can’t quite understand how thrust loads on the secondary shaft are handled to keep it from walking with significant load especially with the Quaife assembly where I was trying to figure out if a spacer was missing as there is nothing on the front bearing on the secondary shaft setting or retaining its position on the shaft other than it being a press fit.

 

Bob, wouldn't the semi-straight-cut design of the Quaife gears add significant strength, regardless of materials used?

 

Yea but they are still 10 to 15% less wide than the 6 speed. the straighter cut bigger tooth design would be way stronger if implemented in an AZ6 6 speed.

 

I'd worry about the straighter cut gears putting even more bending load on the 6 speed shafts.

 

Straight cut doesn't put bending loads in the shaft. it just makes more noise. straight cut tooth geometry maximizes the cross section of material at the base of the tooth to make it stronger. helical teeth can create shaft thrust issues. I've failed two 6speeds and one 5speed mainly due to shaft thrust forces taking out bearings and the shaft walking for/aft.

 

Another observation. the input gears are wider on an NB 5sp than an NA 5sp. I think the NB 5sp is a tad stronger.

 

That has been our experience.